I was wondering the same thing! At the heart of my original question was, "is the digital input section of the HY VFD floating WRT ground?" My quick measurements suggest that it is - 24V between FOR and DCM (to be expected), but a few tenths of a volt between either connection and ground. Measurement with a DMM, so high impedance and that doesn't sound unreasonable. When I look at how I wired the analogue speed signal to the VFD, I see that ACM (analogue "ground") was connected to the control box ground as that was how the analogue output from the motion controller was referenced so floating digital input doesn't seem silly.

I'll be testing this anyway, once I've resolved the little issue of why my stepper drivers don't seem to be responding to the pulse output from the UC300. Probably forgotten a wire somewhere...